Update librato backend to latest Librato bindings.

I've updated librato backend to the latest librato bindings: https://github.com/librato/python-librato.
You already have pull-request from drio(https://github.com/frankwiles/django-app-metrics/pull/31), but his pull-request doesn't fix tasks for the backend.
My fix has natural behavior for metric(...) and gauge(...) methods:
-  metric(...) creates and updates a counter;
-  gauge(...) creates and submits a gauge value.